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Formation of a Porphyrin Monolayer Film by Axial Ligation of Protoporphyrin IX Zinc to an Amino-Terminated Silanized Glass Surface Zhijun Zhang,† Ruisheng Hu, and Zhongfan Liu* Center for Nanoscale Science and Technology (CNST), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China Received August 16, 1999. In Final Form: October 1, 1999 A porphyrin covalently appended monolayer film on a glass substrate prepared by axial coordination reaction of protoporphyrin IX zinc (ZnPP) and a self-assembled monolayer of (3-aminopropyl)trimethoxysilane on a glass surface was reported. The structure and stability of the porphyrin film to photo reaction, acid, and base and its reactivity with imidazole were investigated using X-ray photoelectron spectroscopy and UV-visible spectroscopy.
Introduction 1
In our preceding paper, we described preparation and characterization of a metalloporphyrin, cobalt(II) 5,10,15,20-tetraphenylporphyrin (CoTPP) covalently bonded to 4-pyridinethiolate self-assembled monolayer (SAM) on gold. The main conclusions reached from the paper include the following: (1) The porphyrin-appended monolayer films can be fabricated via axial ligation of the central metal to the N atom of the pyridinethiolate SAM, as evidenced by surfaced enhanced Raman scattering (SERS) result. (2) The porphyrin molecule assumes a nearly flat orientation with respect to the surface of the gold substrate, deduced from the electrochemical measurement. As a part of our systematic investigation of the porphyrin SAMs on metal (gold, silver) and nonmetal (silicon, quartz, glass) surfaces, herein we report on preparation and X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) studies of a SAM film of protoporphyrin IX zinc (ZnPP) on a glass surface. Such work is still quite limited2-6 compared to porphyrin-bonded films on metal surface.7-12 * To whom correspondence should be addressed. Phone and Fax: +86-10-62757157. E-mail:
[email protected]. † Present address: Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan. E-mail:
[email protected]. (1) Zhang, Z. J.; Hou S. F.; Zhu, Z. H.; Liu, Z. F. Langmuir, in press. (2) Xiao, J.; Meyerhoff, M. E. Anal. Chem. 1996, 68, 2818. (3) Li, D.; Moore, L. W.; Swanson, B. I. Langmuir 1994, 10, 1177. (4) Li, D.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975. (5) Pilloud, D. L.; Moser, C. C.; Reddy, K. S.; Dutton, P. L. Langmuir 1998, 14, 4809. (6) Da Cruz, F.; Driaf, K.; Berthier, C.; Lameille, J.-M.; Armand, F. Thin Solid Films 1999, 349, 155. (7) Zak, J.; Yuan, H. P.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (8) (a) Hutchison, J. E.; Postlehwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (b) Postlehwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109. (c) Hutchison, J. E.; Postlehwaite, T. A.; Chen, C.-H.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W. Langmuir 1997, 13, 2143. (9) Gust, D.; Moore, T. A.; Moore, A. L.; Luttrull, D. K.; DeGraziano, J. M.; Boldt, N. J.; Auweraer, M. V.; De Schryver, F, C. Langmuir 1991, 7, 1483. (10) (a) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447. (b) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335. (c) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull. Chem. Soc. Jpn. 1999, 72, 485, and references cited therein.
The formation of a porphyrin overlayer on the aminoterminated glass surface can be achieved through the procedure shown in Scheme 1. Experimental Section Chemicals. Protoporphyrin IX zinc (ZnPP) and (3-aminopropyl)trimethoxysilane (APTMS) were purchased from Aldrich and Sigma, respectively, and used as received. Ethanol, chloroform, tetrahydrofuran (THF), and other organic solvents used were spectroscopic grade. Milli-Q water (>16.0 MΩ‚cm) was used throughout the experiment. Preparation of the Substrates. Prior to film preparation the glass substrates were sonicated in acetone and then in pure water for 5 min, respectively. This was followed by cleaning treatment in a freshly prepared piranha solution (H2SO4:30% H2O2, 3:1 in volume) at 90 °C for 10 min and then by rinsing with a large volume of ultrapure water. After being dried with pure nitrogen gas, the substrate were cured in a drying oven at 100 °C for 30 min. Formation of a Silanized SAM and a Porphyrin Overlayer. Preparation of APTMS SAMs on glass surfaces was performed by immersion of the dry substrates into a dilute solution of APTMS in methanol (5% in volume) for 24 h at room temperature. Then the substrates were rinsed thoroughly with methanol and treated under sonication for 5 min in methanol to remove physisorbed APTMS. After being rinsed with ultrapure water and blown dry with pure N2, the substrates were kept in a drying oven at 120 °C for 30 min. Thus treated amino-terminated silanized substrates were then immersed into a THF-ethanol mixed solution (5:1 (v/v)) of ZnPP (1 × 10-5 M) at ambient temperature for ca. 72 h. Then they were withdrawn and thoroughly rinsed with THF-ethanol mixed solvent and sonicated in the THF-ethanol bath at ambient temperature for 5 min. The chemically modified substrates were then rinsed again with the mixed solvent and blown dry with nitrogen gas. XPS Measurements. X-ray photoelectron spectra were recorded with an ESCALab 220I-XL Surface Microanalysis System equipped with a standard Mg KR1,2 radiation source at 1253.6 eV and 300 W power at the anode. The X-ray spot size was 2.5 mm and the takeoff angle 90°. A low-resolution survey spectrum over 0-1100 eV binding energy range was acquired for one scan. High-resolution spectra of each detected element (11) (a) Guo, L.-H.; Mclendon, G.; Razafitrimo, H.; Gao, Y. L. J. Mater. Chem. 1996, 6 (3), 369. (b) Ashkenasy, G.; Kalyuzhny, G.; Libman, J.; Rubinstein, I.; Shanzer, A. Angew Chem., Int. Ed. Engl. 1999, 38, 1257. (12) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir 1996, 12, 5742.
10.1021/la991106e CCC: $19.00 © 2000 American Chemical Society Published on Web 12/03/1999
Porphyrin Monolayer Film by Axial Ligation of ZnPP Scheme 1
were obtained for up to 10 scans each. The spectra were corrected to C(1s) at 284.7 eV binding energy. UV-Visible Spectra. The UV-vis spectra were taken on a JASCO V550 spectrophotometer. For linear dichroism measurement, a UV dichroic sheet polarizer (Melles Griot) was placed in front of the sample. All UV-vis spectra of porphyrin-coated glass substrates were obtained by using a silanized glass slide as reference.
Results and Discussion Characterization of the Porphyrin Monolayer. (A) XPS Results. XPS technique has proven to be a very useful tool to investigate the composition and structure of the SAMs.3,4,7-15 In the present work XPS was employed to confirm the formation of the porphyrin monolayer on the top of the amino-terminated silanized glass surface. Figure 1 shows the low-resolution survey spectrum of ZnPPamino-terminated SAM on a glass substrate. A single scan XPS spectrum of the Zn region is presented as Figure 2. The peaks due to Zn, N, C, and Si elements clearly appear in the survey spectrum. The peak at 1021.7 eV binding energy for Zn(2P3/2) is in excellent agreement with that in the literature.16 All these results, combined with UVvis data (vide infra), indicate the formation of the ZnPP film on the glass surface. Figure 3 depicts a single scan XPS spectrum of N(1s) of the porphyrin film on the glass slide. The broad overlapped band can be fitted into two peaks, with one at 401.3 eV (A, solid line) and the other at 398.5 eV (B, dashed line) binding energy, respectively. It is noted that the XPS spectrum of the N(1s) region shows no significant difference between the APTMS SAMs with and without porphyrin overlayer. In a paper on XPS investigation of the SAMs of APTMS on a silicon surface, Bierbaum et al.13 observed two different nitrogen species and they assigned the two peaks at 399.3 and 401.1 eV binding energy to free aliphatic amino groups and protonated aliphatic amino groups. Referring to the aforementioned work, it seems reasonable that, in the porphyrin overlayered SAMs on glass surface, the peak at 401.3 eV is due to protonated amino groups, which remained unligated to the Zn ion of the porphyrin molecules. The peak at 398.5 eV may be contributed from several species, the (13) Bierbaum, K.; Kinzler, M.; Woll, C.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (14) Kondo, T.; Yanagida, M.; Shimazu, K.; Uosaki, K. Langmuir 1998, 14, 5656. (15) Ulman, A. An Introduction to Ultrathin Organic Films, from Langmuir-Blodgett Films to Self-Assembly; Academic Press: San Diego, CA, 1991. (16) Barr, L. Modern ESCA: The Principle and Practice of X-ray Photoelectron Spectroscopy; CRC Press: Boca Raton, FL, Ann Arbor, MI, London, and Tokyo, 1994.
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pyrrole nitrogens of the porphyrin molecules,17,18 axial ligand amino group, and/or unreacted free amino group in the silanized SAM. From the present spectrum of the N(1s) region it is difficult to differentiate these species occurring in different binding states. Our interpretation of the XPS result is different from a very recent publication on a porphyrin SAM prepared using an approach similar to ours by Da Cruz et al.6 (B) UV-Vis Spectra. Figure 4 gives UV-vis spectra of ZnPP SAM (dashed line) and ZnPP in THF-ethanol solution (solid line). It can be seen from the spectra that the porphyrin SAM exhibits three bands at 416, 545, and 581 nm, assignable to the Soret band and two Q-bands, respectively.19 The Soret band appears at 417 nm, and two small Q-bands are at 546 and 483 nm for the porphyrin solution. The UV-vis spectra confirm the formation of the porphyrin monolayer on the silanized glass substrate. It is noted that the Soret band for the porphyrin monolayer film shows no marked shift but becomes broad compared to that for the solution. It is very likely that axial ligation and relative rigidity of the porphyrin molecules in the film are responsible for the broadening of the Soret band. There is no clear evidence for the formation of the porphyrin aggregates in the SAM, as aggregation may shift the Soret and Q-bands significantly.4,7,9 Orientation of Porphyrin in the SAMs. Polarized UV-vis spectroscopy is a convenient and useful tool in evaluating molecular orientation in thin solid films such as LB15,20,21 and SAMs.5 To estimate molecular orientation of the porphyrin in the SAM, we performed polarized UVvis spectra with an incident beam at an angle of 45°. The polarized spectra of porphyrin SAM on a glass substrate with the light beam polarized horizontally (out of plane, dotted line) and vertically (in plane, solid line) are shown in Figure 5. The tilt angle of the porphyrin plane with respect to the normal of the substrate is calculated according to the following equation:22
D(λ) ) Ah/Av ) (2 sin2 R sin2 γ)/(2 - sin2 γ) + cos2 R (1) where the d(λ) is the dichroic ratio between the absorption of horizontal (out of plane) (Ah) to vertically (in plane) (Av) polarized light. R is the angle between the incident beam and normal to the substrate; γ, the angle between the normal to the porphyrin plane and the normal to the substrate surface. γ was calculated to be 43°. This result seems unreasonable because it was expected that the porphyrin molecules should assume coplanar orientation to the surface of the substrate.3 We believe that two factors may be responsible for the large tilt angle calculated. First, the porphyrin molecules may not have definite orientation because the sublayer SAM of APTMS on the glass substrate shows disordered structure. A recent study by Bierbaum et al.13 suggests that in the SAM of APTMS on the silicon surface, the APTMS molecules arranged disorderly, as was deduced from their XPS data. It was pointed out by Bierbaum and his colleagues that the (17) Macquet, J. P.; Millard, M. M.; Theophanides, T. J. Am. Chem. Soc. 1978, 100, 4741. (18) Wagner, C. D.; Riggs, W. M.; Davis, I. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: New York, 1979. (19) Dolphin, D., Ed. The Porphyrins; Academic Press: New York, 1978. (20) Schick, G. K.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (21) Vandevyer, M.; Barraud, A.; Raudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (22) Blasie, J. K.; Erecinska, M.; Sanmuels, Leigh, J. S. Biochim. Biophys. Acta 1978, 501, 33.
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Figure 1. XPS survey spectrum of a ZnPP monolayer film on the silanized glass substrate.
Figure 2. Single scan XPS spectrum of the Zn(2p) region of the ZnPP monolayer film.
observed disorder in the APTMS silanized silicon surface is due to interactions of the APTMS molecules (free amino group and their protonated derivatives) and repulsive dipole and dipole interactions between the amino groups. Lack of efficient hydrophobic interactions between short alkyl chains in the APTMS SAM, we think, may also be an important factor which causes the disorder of the APTMS monolayer films, as in the case of LB films of fatty acids with short length.15 The second reason for the
large tilt angle of porphyrin plane to the glass surface is that the aforementioned equation holds only if the distribution for the tilt angle is very narrow. Therefore it seems reasonable that eq 1 may not apply to our porphyrin monolayer films. Stability of the Porphyrin SAM. SAMs are featured by their superior mechanical and thermal stabilities over other thin solid films. The resistance of the functionalized SAMs to photoreaction, acid-base, and others are also
Porphyrin Monolayer Film by Axial Ligation of ZnPP
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Figure 3. Single scan XPS spectrum of the N(1s) region of the porphyrin film. It can be fitted into two peaks: A, solid line, at 401.3 eV binding energy; B, dashed line, at 398.5 eV binding energy.
Figure 4. UV-vis spectra of a ZnPP monolayer film on the silanized glass substrate (dashed line) and ZnPP in THFethanol solution (solid line).
importance for their practical applications.15 We monitored the reactivities of the porphyrin SAM to photoreaction, acid-base, and imidazole by employing UV-vis spectroscopy. (A) UV-Visible Light. We measured the UV-vis spectra of the porphyrin SAMs on the glass slides freshly prepared and kept for 2 weeks at ambient condition under natural light and the porphyrin films with and without UV light (365 nm) irradiation for 10 min (data not shown). No noticeable photoinduced changes were observed in the UV-vis spectra, suggesting that the film is photostable. (B) Acid and Base. Immersion of the porphyrin SAM into HCl aqueous solution (0.1 M) and NaOH aqueous solution (0.1 M) for 10 min, respectively, imposes no appreciable changes in the UV-vis spectra of the por-
Figure 5. Polarized UV-vis spectra of the ZnPP monolayer film on the silanized glass substrate with the light beam polarized horizontally (out of plane, solid line) and vertically (in plane, dotted line). The incident light is at an angle of 45°.
phyrin film (data not shown), indicating acid and base do not react with the central metal and the macrocycle of the porphyrin molecules under the present experimental condition. (C) Reactivity of the Porphyrin SAM with Imidazole. The porphyrin monolayer-coated silanized glass substrates were immersed into ethanol solution of imidazole (1 × 10-3 M) for 24 h at room temperature (ca. 25 °C). Then it was withdrawn and thoroughly rinsed with ethanol. UV-vis spectra of the porphyrin film were taken before (solid line) and after (dotted line) samples were treated with imidazole solution, as is shown in Figure 6. There are no marked changes observed for the porphyrin film treated with imidazole solution. This demonstrates that
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the amino group in the fifth axial ligation site occurred. It is known from study on axial ligation reaction in solutions19 that imidazole usually has a stronger propensity than aliphatic amino groups to coordinate with metalloporphyrin derivatives. It is likely that the relative rigidity of the porphyrins and stereohindrance are responsible for nonoccurring of the replacing of the amino by imidazole coordinated as the fifth ligand to the core metal ion, Zn ion, of the porphyrin molecules in the SAMs. Conclusion
Figure 6. UV-vis spectra of the ZnPP monolayer film before (solid line) and after (dotted line) immersion into an ethanol solution of imidazole for 24 h at room temperature (ca. 25 °C).
neither ligation in the sixth axial site, in good agreement with the result reported by Pilloud et al.,5 nor replacing
Axial ligation of ZnPP to the amino-terminated glass surface produces a very stable porphyrin monolayer film. The polarized UV-vis measurement suggests that the porphyrin molecules were oriented somewhat disorderly in the film, due to the disordered structure of the APTMS SAM sublayer on the glass surface. Further work to improve the packing and orientation of coupling layers, on the top of which porphyrin molecules will be covalently bonded, and then, consequently, a well-ordered porphyrin monolayer film can be formed, is in progress now. LA991106E
